Kwon Alexander is done for the season, per the ever-intrepid Adam Schefter of ESPN.
Alexander, a key cog in the 49ers' electric defense, got hurt in the third quarter of the club's 28-25 win over the Cardinals on Thursday Night Football. With Alexander sidelined, rookie Dre Greenlaw is poised to step into the veteran's spot at weakside linebacker alongside Fred Warner in the Niners linebacking corps.
Greenlaw was a fifth-round pick last spring.
